Sodium thiopental in the comatogenic (but not soporogenic) dose caused hyperammoniemia in rats. Blood ammonium level increased 3-fold within 3 h and 5-fold within 18 h. Blood urea level increased by one-third within 18 h against the background of unchanged creatinine level and hematocrit. Urinary excretion of ammonium did not decrease, while its release with exhaled air increased, indicating intensification of ammonium formation in the body. Barbiturate coma did not change the slope of curves of dose-dependent increase of ammonium or urea levels in the blood of rats injected with ammonium acetate, which attested to the absence of appreciable disorders in the ammonium detoxifying function of the liver. Ammonium hyperproduction could be caused by gastrointestinal stasis verified by X-ray examination and confirmed by correlation between blood urea level and stool retention in narcotized rats.